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Results and discussion 17 A B C D 0 2 3 10 at E F ec G H AL AL en * * * 18 24 30 36 I se AL ML PL J se se se Lo 75 hpf Pe 360 hpm se Figure 1. Developmental stages of Terebratalia transversa at a water temperature of 11.5 °C. Numbers indicate the age in hours after fertilization (hpf) for all stages except of J where it is hours after the onset of metamorphosis (hpm). Size of all stages is around 120 µm in diameter, except for J where it is around 200 µm. Anterior is oriented upwards and cilia are omitted for clarity. (A) unfertilized oocyte (black) with an egg shell (grey). (B) Lateral view of two cell stage with two polar bodies and the egg shell (grey). (C) Apical view of a four cell stage. (D) Sagittal section through an early blastula. (E) Sagittal section through a late blastula at the onset of gastrulation. (F) Gastrula with ectoderm (ec), endoderm (en), and blastopore (asterisk). The gastrula starts to swim at this point of development. (G) Elongated late gastrula with slit-like blastopore (asterisk) and first signs of a distinguished apical lobe (AL). (H) Larva with further developed lobes, almost closed blastopore (asterisk), and apical tuft (at). (I) Fully established larva with apical lobe (AL), mantle lobe (ML), and pedicle lobe (PL). Four sets of setae bundles (se, only two visible) originate from the mantle lobe. (J) Juvenile with lophophore (Lo), and pedicle (Pe). The remaining larval setae (se) extend beyond the two valves. se Novocrania anomala, a representative of Craniiformea Development of Novocrania anomala and regional specification during embryogenesis has been described previously (Nielsen 1991; Freeman 2000). My results are congruent with these data. However, the two authors disagree about the development of the coelom and the formation of the mesoderm. According to Nielsen, the sheet of cells that invaginates during gastrulation is composed of two cell populations, endoderm and mesoderm, whereas Freeman states that the mesoderm is formed by individual cells which immigrate from the endodermal cell layer after invagination has been completed (Nielsen 1991; Freeman 2000). Nielsen describes the coelom as consisting of an anterior coelomic pouch in the apical lobe and three pairs of coelomic cavities in the
18 Results and discussion posterior lobe of the larva, whereas Freeman denies the existence of larval coelomic structures and states that the coelom develops after the larvae have undergone metamorphosis (Nielsen 1991; Freeman 2000). The methods used here do not allow a conclusive statement concerning coelom and mesoderm formation in larvae of N. anomala, there is more work needed to resolve the controversies on an ultrastructural level. Cleavage is radial and the first two divisions are holoblastic (Fig. 2B). The gastrula is first spherical and invagination takes place at the vegetal pole of the larva. The archenteron cells come to lie opposite of the ectoderm. Subsequently, the blastocoel disappears completely (Fig. 2C). Later in development the gastrula elongates and the blastopore comes to lie at the postero-ventral side of the swimming larva (Fig. 2D). The elongated gastrula subsequently differentiates into two larval lobes, an apical lobe and a posterior lobe (Fig. 2E, F). Larval development completes with the growth of three pairs of dorsal setal bundles on the posterior lobe (Fig. 2G). Prior to settlement, the larva swims along the bottom of the culture dish, probably in order to test if the substrate is suitable for settlement. In contrast to the descriptions by Nielsen (1991), the larvae do not curl before metamorphosis. Although curled larvae are found in the culture dishes, these seem to be unable to metamorphose. What causes the curling is unclear, however it can clearly be seen in the musculature of settled larvae that the remaining larval muscles are elongated and relaxed in contrast to the contracted musculature of curled larvae (Fig. 3A, B, and Chapter III). At a water temperature of 14 °C, metamorphosis takes place around six to ten days after fertilization (dpf). During metamorphosis the larva attaches to the substrate, secretes the shell, and retains its larval lobes, which are subsequently transformed and form the lophophore and other adult organs (Figs. 2H, 3B, C).
Page 1 and 2: Comparative neurogenesis, muscle de
Page 3 and 4: 2 Principal supervisor Assoc. P
Page 5 and 6: 4 Preface This thesis presents
Page 7 and 8: 6 Abstract Brachiopods are a sm
Page 9 and 10: 8 Acknowledgements The endeavou
Page 11 and 12: 10 Introduction Nervous system Micr
Page 13 and 14: 12 Material and methods Material an
Page 15 and 16: 14 Material and methods purpuratus,
Page 17: 16 Results and discussion Results a
Page 21 and 22: 20 Results and discussion Myogenesi
Page 23 and 24: 22 Results and discussion Wanninger
Page 25 and 26: 24 Results and discussion Growth pa
Page 27 and 28: 26 Results and discussion argument
Page 29 and 30: 28 References References Abdelkhale
Page 31 and 32: 30 References priapulids and protos
Page 33 and 34: 32 References James, M. A., Ansell,
Page 35 and 36: 34 References regionalization, prol
Page 37 and 38: 36 References 211. Williams, A., Ca
Page 39 and 40: 38 Chapter II Frontiers in Zoology
Page 53 and 54: 52 Chapter III Chapter III Altenbur
Page 55 and 56: 54 Chapter III Altenburger and Wann
Page 63 and 64: 62 Chapter IV Chapter IV Altenburge
Page 65 and 66: 64 Submitted manuscript Chapter IV
Page 67 and 68: 66 Submitted manuscript Chapter IV
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68 Submitted manuscript Chapter IV
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